Strategies for quantification and confirmation of multi-class polar pesticides and transformation products in water by LC–MS2 using triple quadrupole and hybrid quadrupole time-of-flight analyzers

Liquid chromatography–tandem mass spectrometry (LC–MS2) is an excellent technique for the determination of polar pesticides and transformation products in water. On-line trace enrichment by solid-phase extraction (SPE)-LC in combination with MS2 satisfies most analytical demands of modern laboratories for pesticide-residue analysis, with respect to sensitivity (limits of detection and limits of quantification at sub-μg/L levels), selectivity, reliability and rapidity, and all with very little sample manipulation. However, several obstacles have to be overcome when developing SPE-LC–MS2 multi-residue and confirmative methods. The different physicochemical characteristics and wide polarity range of pesticides and their transformation products make their simultaneous preconcentration, chromatographic separation and determination difficult. Also, for the correct confirmation of the identity of compounds, it is necessary to increase the number of monitored MS transitions, which normally leads to a loss in sensitivity. This may hamper confirmation at low concentrations in a single chromatographic run (i.e., when a second analysis is not performed). In this article, we discuss different strategies for developing sensitive multi-residue methods for pesticides in water, able to correctly quantify and confirm compounds detected in samples. We discuss the use of different analyzers, triple quadrupole and hybrid quadrupole time-of-flight and compare their performances for screening purposes, and for the quantification and confirmation of positive samples. We present several practical cases of water samples, including some positive findings that would result in false positives if no criteria were applied for unequivocal confirmation of the identity of detected compounds.